Copying machines of certain type and original document reading devices are adapted to illuminate an original in the form of a slit and transmit an image of the original to a photosensitive member. With such devices, the original may be moved relative to the illumination system which is at rest, or a portion of the illumination system, as well as of the image transmitting means, may be moved under a stationary transparent glass plate having the original placed thereon. In either case, illumination systems of the construction shown in FIG. 1 are most widely used.
With reference to FIG. 1, an original 10 is placed on a transparent glass plate 11, and a mirror 13 for transmitting an image of the original 10 is disposed immediately below the portion 12 of the original 10 to be illuminated. The image of the original is directed from the illuminated portion 12 to an unillustrated photosensitive member through the mirror 13, etc. A light source 1 and elliptical mirrors 2, 3 are provided on one side of the optical path OP, and a plane mirror 4 is disposed on the other side.
The first elliptical mirror 2 is shaped along part of a first ellipse E1. The first ellipse E1 has a major axis MA1 extending through the light source 1 and the illuminated portion 12 and has its first focus F.sub.11 positioned at the center of the light source 1 and its second focus F.sub.12 within the transparent glass plate 11.
The second elliptical mirror 3 is shaped along part of a second ellipse E2. The first focus F.sub.21 of the second ellipse E2 coincides with the first focus F.sub.11 of the first ellipse E1, while the major axis MA2 thereof through the first focus F.sub.21 and its second focus F.sub.22 extends toward the plane mirror 4. The second ellipse E2 is so positioned that the image of the second focus F.sub.22 formed by the plane mirror 4 will coincide with the second focus F.sub.12 of the first ellipse E1 which is positioned within the glass plate 11. The image of the second ellipse E2 formed by the plane mirror 4 is illustrated as an ellipse E2'.
With the above arrangement, the bundle of rays emitted from the light source 1 and reflected from the first elliptical mirror 2 passes through a light path, which is shown by leftwardly upward hatching, on the left side of the optical path OP for the image of the original and is directed directly toward the illuminated portion 12. On the other hand, the bundle of rays emitted from the light source 1 and reflected from the second elliptical mirror 3 crosses the optical path OP for the image of the original and is then reflected again from the plane mirror 4 and directed to the illuminated portion 12 through a light path which is shown by rightwardly upward hatching on the right side of the optical path OP. In this way, the original 10 is illuminated from opposite sides of the optical path OP for the original image in order to prevent the shadows of edges of the original from appearing as black lines in the resulting copy. The ellipses E1 and E2 are further so arranged that the bundles of rays will be incident on the illuminated portion at a smaller angle to thereby achieve an improved illumination efficiency. For example, if the second elliptical mirror 3 is formed along the first ellipse E1, the bundle of rays reflected therefrom will be incident on the illuminated portion at a larger angle to lower the illumination efficiency, in other words, the ratio of the amount of light actually reaching the photosensitive member as the original image to the amount of light illuminating the original.
It is now assumed that at the center of the light source 1, i.e., at the first foci F.sub.11, F.sub.12 of the ellipses E1, E2, angles .alpha., .beta. are subtended by the first elliptical mirror 2 and the second elliptical mirror 3, respectively. The light emitted from the center of the light source then reaches the mirrors 2, 3 within these angular ranges .alpha., .beta., respectively. According to the definition of the ellipse, the light is reflected toward the individual second foci. However, the portions of the two elliptical mirrors 2, 3 behind the light source 1 which subtend angles .gamma., .delta. at the center of the light source do not actually contribute to the illumination of the original because the light within these ranges of angles .gamma., .delta. are reflected from the mirrors and returned to the light source 1, where the light is diffused, absorbed and refracted by the tubular surface, filament, etc. of the light source 1.
In the case of such a system designed by the present inventor wherein .alpha.=125.degree. and .beta.=139.degree., the angles .gamma., .delta. were as large as 36.degree. and 61.degree., respectively. In this case, the apparent angle of reflection (.alpha.=.beta.) is 264.degree., whereas the effective angle of reflection (.alpha.=.beta.-.gamma.-.delta.) is as small as 167.degree.. Thus the illumination system is very low in illumination efficiency.